Programmable infusion pumps for delivering nutritional fluids and medicine to patients in accordance with predetermined fluid delivery parameters are in wide usage. One type of infusion pump is a peristaltic pump arranged along flexible connective tubing carrying fluid from a fluid source to the patient. The peristaltic pump has a pumping mechanism for progressively squeezing successive portions of the tubing to cause fluid to flow through the tubing in a flow direction toward the patient. In a common arrangement, the pumping mechanism includes a motor-driven wheel having radial fingers or rollers that engage a segment of the tubing arranged about a circumferential portion of the wheel. As the wheel rotates, fluid is pumped through the tubing to the patient. The tubing segment arranged about the pump wheel may be held in a U-shaped configuration by a cassette designed for receipt in a channel or receptacle area of the pump. The cassette may provide terminals for connecting an incoming line of tubing coming from the fluid source and an outgoing line of tubing going to the patient to opposite ends of the U-shaped tubing segment received by the pump. In the present specification, the terms “upstream” and “downstream” are in reference to the direction of fluid flow caused by the pumping mechanism. For example, the incoming line of tubing is “upstream” from the pumping mechanism, and the outgoing line of tubing is “downstream” from the pumping mechanism.
A recognized concern, especially when pumping viscous nutritional fluids for enteral feeding, is the formation of blockages (“occlusions”) within the tubing that may reduce or completely prevent flow. As a safety measure, it is known to provide a pair of occlusion sensors on the infusion pump. An upstream occlusion sensor is arranged to engage the tubing at an upstream location relative to the pumping mechanism, and a downstream occlusion sensor is arranged to engage the tubing at a downstream location relative to the pumping mechanism. The occlusion sensors may include transducers or strain gauges that detect deflection of the flexible tubing wall caused by a local pressure differential (either pressure increase or decrease) relative to an equilibrium fluid pressure within the tubing and provide an electronic signal indicative of the deflection. For example, if an occlusion forms at a location in the downstream tubing between the pump and the patient, a bulge or outward deflection of the tubing wall will be detectable by the downstream occlusion sensor. Conversely, if an occlusion forms at a location in the upstream tubing between the fluid source and the pump, the continued operation of the pumping mechanism creates a vacuum between the occlusion location and the pumping mechanism and an inward deflection of the tubing wall will be detectable by the upstream occlusion sensor.
The signals from the upstream and downstream occlusion sensors are monitored and compared to respective signal baselines to detect occlusion. The upstream sensor signal baseline is the signal provided by the upstream sensor corresponding to a condition of fluid pressure equilibrium at the upstream sensor location. Likewise, the downstream sensor signal baseline is the signal provided by the downstream sensor corresponding to a condition of fluid pressure equilibrium at the downstream sensor location. The upstream and downstream baselines may be established by an initialization routine executed when the pump is started up. During pump operation for infusion, respective differences between the upstream sensor signal and upstream baseline, and between the downstream sensor signal and downstream baseline, are monitored. If a difference between the sensor signal and the corresponding baseline exceeds a predetermined threshold for a predetermined period of time, an upstream occlusion is detected. As will be understood, establishing and maintaining valid baselines for the upstream and downstream occlusion sensors is essential for proper occlusion detection.
One situation where an invalid baseline may inadvertently be used occurs when an occlusion is detected, pump operation is stopped, and a door of the pump is opened to access the cassette and tubing to permit replacement of the occluded tubing. If new tubing is not installed and the pump is restarted with the occluded tubing still installed, the initialization routine may be fooled into using the pressurized tubing to establish new baselines. This problem is referred to in the art as baseline “ratcheting.”
Sensor drift may also interfere with proper occlusion detection. During infusion protocols having a very low infusion rate, the pump motor may actually run for very short period of time (e.g., one stepper motor “tick” or increment per minute). When the pump motor is not running, it may be assumed that increases in the downstream sensor signal are due to sensor drift, not to an actual build up in pressure caused by an occlusion in downstream tubing. Similarly, when the pump motor is not running, it may be assumed that decreases in the upstream sensor signal are due to sensor drift, not to an actual pressure decrease caused by an occlusion in upstream tubing. If the changes attributable to sensor drift are included in calculating the difference relative to the associated sensor baseline, false occlusion alarms may occur.